ES2859525T3 - Metal clad steel strip and its manufacturing procedure - Google Patents

Abstract

An Al-Zn-Si-Mg alloy coated steel strip comprising an Al-Zn-Si-Mg alloy coating on a steel strip, with a coating thickness greater than 7 microns and less than 30 microns, the coating having coating thickness variations of no more than 40% in any 5 mm diameter section of the coating, with the alloy consisting of 40 to 60% by weight aluminum, 40 to 60% zinc , 0.3 to 3% silicon and 0.3 to 10% magnesium as main elements, and optionally more than 250 ppm of strontium and less than 3000 ppm of strontium, and optionally iron, vanadium and chromium, and other elements that are present as unavoidable impurities, the microstructure of the coating comprising Mg2Si particles, and the distribution of the Mg2Si particles being such that there is no more than 10% by weight of Mg2Si particles on the surface of the coating.

Description

DESCRIPTION

Metal clad steel strip and its manufacturing procedure

The present invention relates to a steel strip having a corrosion resistant metal alloy coating.

The present disclosure more particularly relates to the general field of a corrosion resistant metal alloy coating containing aluminum-zinc-silicon-magnesium as the main elements of the alloy, and hereinafter referred to as "Al-Zn-Si-alloy. Mg "on this basis. Those alloy coatings may contain other elements that are present as deliberate alloy additions or as unavoidable impurities. Therefore, the phrase "Al-Zn-Si-Mg alloy" is understood to encompass alloys that contain those other elements and the other elements may be deliberate alloy additions or unavoidable impurities.

Typically, the Al-Zn-Si-Mg alloy comprises the following ranges in% by weight of the elements aluminum, zinc, silicon and magnesium:

Typically, the corrosion resistant metal alloy coating is formed on the steel strip by a hot dip coating process.

In the conventional hot dip metal coating process, the steel strip generally passes through one or more heat treating furnaces and then through a molten metal alloy bath which is held in a coating pan. The heat treatment furnace adjacent to the lining tank has an outlet conduit that extends downwardly to a location below the upper surface of the bath.

The metal alloy is normally kept molten in the investment tank through the use of heating inductors. The strip usually exits heat treatment furnaces through a final outlet section in the form of an elongated furnace outlet channel or conduit which is immersed in the bath. Inside the bath, the strip passes around one or more pot rollers and is pulled up from the bath and coated with the metal alloy as it passes through the bath.

After exiting the coating bath, the metal alloy coated strip passes through a coating thickness control station, such as a gas cleaning station or gas knife, where its coated surfaces are subjected to jets of gas from cleaning to control the thickness of the coating.

The metal alloy coated strip then passes through a cooling section and is subjected to forced cooling.

The cooled metal alloy coated strip can optionally be post-conditioned by passing the coated strip successively through a finish roll section (also known as a quench roll section) and a stress leveling section. The conditioned strip is wound on a winding station.

A 55% Al-Zn alloy coating is a known metal alloy coating for steel strip. After solidification, a 55% Al-Zn alloy cladding typically consists of a-Al dendrites and a p-Zn phase in the interdendritic regions of the cladding.

Silicon is known to be added to the coating alloy composition to prevent over-alloying between the steel substrate and the molten coating in the hot dip coating process. Some of the silicon participates in the formation of a quaternary alloy layer, but most of the silicon precipitates as needle-shaped pure silicon particles during solidification. These needle-shaped silicon particles are also present in the interdendritic regions of the coating.

Applicant has found that when Mg is included in a 55% Al-Zn-Si alloy coating composition, Mg produces certain beneficial effects on product performance, such as better cut edge protection, when changing the nature of the corrosion products that are formed.

However, Applicant has also found that Mg reacts with Si to form a Mg ² Si phase and that the formation of the Mg ² Si phase compromises the aforementioned beneficial effects of Mg in various ways.

One particular form that the present invention focuses on is a surface defect called "speckling." Applicant has discovered that speckling can occur in Al-Zn-Si-Mg alloy coatings under certain solidification conditions. The mottling is related to the presence of the Mg ² Si phase on the surface of the coating.

More specifically, speckling is a defect in which large numbers of coarse Mg ² Si particles clump together on the surface of the coating, resulting in a stained surface appearance that is not aesthetically acceptable. More specifically, the ^{clumped Mg 2} Si particles form darker regions approximately 1-5 mm in size and introduce a non-uniformity in the appearance of the coating, rendering the coating unsuitable for applications where a coating is important. uniform appearance.

The present invention generally provides a strip coated with an Al-Zn-Si-Mg alloy having Mg ² Si particles in the microstructure of the coating, the distribution of the Mg ² Si particles being such that the surface of the coating does not It has more than 10% by weight of Mg ² Si particles on the surface of the coating.

Applicant has discovered that the ^{above-described Mg 2} Si particle distribution in the coating microstructure provides significant advantages and can be achieved by one or more of:

a) strontium additions in the cladding alloy,

b) selecting the rate of cooling during solidification of the coated strip for a given coating mass (ie, coating thickness) coming out of a coating bath; and

c) minimization of coating thickness variations.

Applicant has found that the Sr additions described in more detail below control the Mg ² Si phase distribution characteristics in the thickness direction of an Al-Zn-Si-Mg alloy coating, so that the The surface of the coating has only a small proportion of Mg ² Si particles or is at least substantially free of Mg ² Si particles, so there is a considerably lower risk of speckle by Mg ² Si.

In particular, the Applicant has discovered that when 250-3000 ppm of Sr. is added to a coating bath containing an Al-Zn-Si-Mg alloy, the distribution characteristics of the Mg ² Si phase in the direction The thickness of the coating is completely modified by this addition of Sr from the distribution that is present when there is no Sr in the coating bath. Specifically, Applicant has found that these Sr additions promote the formation of a coating surface that has only a small proportion of Mg ² Si particles or is free of any Mg ² Si particles and consequently considerably less risk. of mottling on the surface.

Applicant has also discovered that the selection of the cooling rate during solidification of a coated strip exiting a coating bath below a threshold cooling rate, typically below 80 ° C / sec. for coating masses of less than 100 grams per square meter strip surface per side, control the Mg ² Si phase distribution characteristics so that the surface has only a small proportion of Mg ² Si particles or is substantially free of Mg ² Si particles, so there is considerably less risk of speckle by Mg ² Si.

Applicant has also found that minimizing coating thickness variations controls Mg ² Si phase distribution characteristics, so that the surface has only a small proportion of Mg ² Si particles or is at least substantially free of Mg ² Si particles, so there is considerably less risk of speckle by Mg ² Si. As in the case of adding Sr and selecting the cooling rate during solidification, the resulting coating microstructure is advantageous in terms of appearance, higher corrosion resistance, and better coating ductility.

EP 1225246, which is considered to represent the closest state of the art, discloses a hot dip coating process for forming a corrosion resistant Al-Zn-Si-Mg alloy on a steel strip.

According to a first aspect of the present invention there is provided an Al-Zn-Si-Mg alloy coated steel strip according to claim 1.

The optional addition of Sr promotes the formation of the above distribution of Mg ² Si particles in the coating.

Preferably the coating contains more than 500 ppm of Mr.

Preferably the coating contains more than 1000 ppm of Mr.

Preferably there are minimal variations in the thickness of the layer.

According to the present invention, there is also provided a hot dip coating process for forming a corrosion resistant Al-Zn-Si-Mg alloy coating on a steel strip according to claim 2.

Preferably the coating contains more than 500 ppm of Mr.

Preferably the coating contains at least 1000 ppm of Mr.

In either situation, the selection of the required cooling rate is related to the thickness of the coating (or the mass of the coating).

Typically, the method comprises selecting the cooling rate to be at least 11 ° C / sec.

By way of example, for a coating with an average thickness of 22 pm, during solidification preferably the cooling rates are as follows:

a) 55 ° C / second in a temperature range of 600-530 ° C,

b) 70 ° C / second in a temperature range of 530-500 ° C, and

c) 80 ° C / second in a temperature range of 500-300 ° C.

The coating bath and the coating on the coated steel strip in the bath may contain Mr.

Preferably the variation in the thickness of the coating should not be greater than 30% in any 5mm diameter section of the coating.

In any given situation, the selection of an appropriate thickness variation is related to the thickness of the coating (or the mass of the coating).

By way of example, for a coating thickness of 22pm, preferably the maximum thickness in any region of the coating greater than 1mm in diameter should be 27pm.

The hot dip coating process can be the conventional process described above or any other suitable process.

Among the advantages of the invention are the following.

• Elimination of the speckle defect and improvement of the production rate the first time. The risk of speckled defect is at least substantially eliminated and the resulting coating surface maintains a beautiful silver metallic appearance. As a result, the quality production rate is improved in the first time and profitability is increased.

• Preventing the speckle defect by adding Sr allows the use of higher cooling rates, reducing the length of cooling equipment required after the pot.

Example

Applicant has carried out laboratory experiments on a series of 55% Al-Zn-1.5% Si-2.0% Mg alloy compositions having up to 3000 ppm Sr coated on steel substrates.

The purpose of these experiments was to investigate the impact of Sr on the surface mottling of coatings. In figure 1 the results of a set of experiments carried out by the applicant that illustrate the present invention are summarized.

The left side of the figure comprises a top plan view of a coated steel substrate and a cross section through the coating with an alloy of 55% Al-Zn-1.5% Si-2.0% Mg without Mr. The coating was not formed taking into account the selection of the cooling rate during solidification and the coating thickness variations that have been discussed above.

The mottling that results from such a coating composition is identified by the arrow in the top plan view. It is evident from the cross section that the Mg ² Si particles are distributed throughout the thickness of the coating. This is a problem for the reasons stated above.

The right side of the Figure comprises a top plan view of a coated steel substrate and a cross section through the coating, with the coating comprised of a 55% Al-Zn-1.5% alloy. Yes 2.0% Mg and 500 ppm Sr. A complete absence of mottling is evident in the top plan view. In addition, the cross section illustrates the upper and lower regions on the surface of the coating and at the interface with the steel substrate that are completely free of Mg ² Si particles, with the Mg ² Si particles being confined to a central strip of the coating. . This is advantageous for the reasons stated above.

The photomicrographs in the Figure clearly illustrate the benefits of adding Sr to an Al-Zn-Si-Mg coating alloy.

Laboratory experiments found that the microstructure shown on the right hand side of the figure was formed with additions of Sr in the range of 250-3000 ppm.

The applicant has also carried out line tests on an alloy composition of 55% Al-Zn-1.5% Si-2.0% Mg (containing no Sr) coated on steel substrates.

The purpose of these tests was to investigate the impact of cooling rates and coating masses on surface mottling of coatings.

The tests covered a coating mass range of 60 to 100 grams per square meter of surface per strip side, with cooling rates up to 90 ° C / sec.

The applicant discovered two factors affecting the microstructure of the coating, in particular the distribution of the Mg ² Si particles in the coatings, in the tests.

The first factor is the effect of the cooling rate of the strip exiting the coating bath before the complete solidification of the coating. The applicant found that controlling the cooling rate makes it possible to avoid mottling.

As an example, the applicant found that for a coating of class AZ150 (or 75 grams of coating per square meter of surface per side of the strip - see Australian standard AS1397-2001), if the cooling rate is greater than 80 ° C / sec., Mg ² Si particles are formed on the surface of the coating. In particular, when the cooling rate was greater than 100 ° C / sec, speckling occurred.

The applicant also found that for the same coating it is undesirable for the cooling rate to be too low, in particular below 11 ° C / sec, since in this case the coating develops a defective "bamboo" type structure, in where the zinc-rich phases form a straight vertical corrosion path from the coating surface to the steel interface, compromising the corrosion performance of the coating.

Therefore, for an AZ150 class coating, under the experimental conditions tested, the cooling rate must be controlled to be in the range of 11-80 ° C / second to avoid mottling on the surface.

On the other hand, the Applicant also found that for an AZ200 class coating, if the cooling rate was higher than 50 ° C / sec, Mg ² Si particles were formed on the surface of the coating and speckling occurred.

Therefore, for an AZ200 class coating, under the experimental conditions tested, a cooling rate in a range of 11-50 ° C / sec is desirable.

The second important factor discovered by the applicant is the uniformity of the coating thickness on the surface of the strip.

The applicant determined that the surface coating of the strip normally exhibited thickness variations that are: a) long-range (along the entire width of the strip, measured by the "weight-strip-weight" procedure in a 50 mm diameter disk) and b) short-range (along each 25 mm of length in the direction of the width of the strip, measured in the cross-section of the coating under a microscope with a magnification of 500x). In a production situation, the long range thickness variation is normally regulated to meet the minimum coating mass requirements defined in the relevant national standards. In a production situation, as far as the applicant is aware, there is no regulation for short-range thickness variation, provided that the minimum coating mass requirements defined in the relevant national standards are met.

However, the applicant found that short range coating thickness variations could be very high, and special operational measures had to be applied to keep the variations under control. In experimental work it was not uncommon for the coating thickness to change by a factor of two or more in a distance as short as 5 mm, even when the product perfectly met the minimum coating mass requirements defined in the relevant national standards. This short range coating thickness variation had a pronounced impact on the Mg ² Si particles on the surface of the coatings.

By way of example, Applicant found that for an AZ150 class coating, even in the desirable cooling rate ranges as described above, if the short-range coating thickness variation was greater than 40% above the thickness nominal coating within a distance of 5 mm across the surface of the strip, Mg ² Si particles were formed on the surface of the coating and thus increased the risk of mottling.

Therefore, under the experimental conditions tested, the short-range coating thickness variation should be controlled to no more than 40% above the nominal coating thickness within a distance of 5mm across the surface of the strip to avoid mottling.

The research work carried out by the applicant on the solidification of Al-Zn-Si-Mg coatings, which is extensive and described in part above, has helped the applicant to understand the formation of the Mg ^{2 phase} If in a coating and the factors that affect its distribution in it. Although the applicant does not wish to be bound by the following discussion, it is this understanding that follows.

When an Al-Zn-Si-Mg alloy cladding is cooled to a temperature close to 560 ° C, the a-Al phase is the first phase to nuclear. The a-Al phase then grows into a dendritic form. As the a-Al phase grows, Mg and Si, along with other soluble elements, are rejected in the molten liquid phase and thus the molten liquid remaining in the interdendritic regions is enriched in Mg and Si.

When the enrichment of Mg and Si in the interdendritic regions reaches a certain level, the Mg ² Si phase begins to form, which also corresponds to a temperature around 465 ° C. For simplicity, an interdendritic region near the outer surface of the cladding will be assumed to be region A and another interdendritic region near the quaternary intermetallic alloy layer on the surface of the steel strip is region B. It will also be assumed that the Enrichment level in Mg and Si is the same in region A as in region B.

At 465 ° C or lower, the Mg ² Si phase has the same tendency to nucleate in region A as in region B. However, the principles of physical metallurgy teach us that a new phase will preferentially nucleate at a site in that the free energy of the resulting system is the minimum. The Mg ² Si phase would normally nuclear preferentially into the B-region quaternary intermetallic alloy layer, provided the coating bath does not contain Sr (the role of Sr with Sr-containing coatings is discussed later). The applicant considers that this is in accordance with the principles stated above, in the sense that there is some similarity in the structure of the crystal lattice between the quaternary intermetallic alloy phase and the Mg ² Si phase, which favors the nucleation of the Mg ² Si phase by minimizing any increase in the free energy of the system. In comparison, for the Mg ² If phase to nucleate in the surface oxide of the coating in region A, the increase in free energy of the system would have been greater.

By nucleating in region B, the Mg ² Si phase grows upwards, along the channels of molten liquid in the interdendritic regions, towards region A. On the growth front of the Mg ² Si phase (region C), the molten liquid phase is depleted in Mg and Si (depending on the partition coefficients of Mg and Si between the liquid phase and the Mg ² Si phase), compared to that of region A. Thus, a diffusion pair is formed between region A and region C. In other words, Mg and Si from the molten liquid phase will diffuse from region A to region C. Note that growth of the a- Al in region A means that region A is always enriched in Mg and Si and the tendency of the Mg ² Si phase to nucleate in region A always exists because the liquid phase is "subcooled" with respect to the Mg phase. ² Yes.

Whether the Mg ² Si phase is nucleated in region A, or whether Mg and Si continue to diffuse from region A to region C, will depend on the level of enrichment of Mg and Si in region A, relevant for the local temperature, which in turn depends on the balance between the amount of Mg and Si that is rejected in that region by the growth of a-Al and the amount of Mg and Si that moves away from that region by diffusion. The time available for diffusion is also limited as the Mg ² Si nucleation / growth process has to be completed at a temperature of around 380 ° C, before the eutectic L ^ Al-Zn reaction takes place, in where L represents the molten liquid phase.

The applicant has found that controlling the balance between the time available for diffusion and the diffusion distance for Mg and Si can control the nucleation or subsequent growth of the Mg ² Si phase or the final distribution of the Mg 2 Si phase. Mg ² Si in the direction of the coating thickness.

In particular, the applicant has discovered that for a given coating thickness, the cooling rate must be regulated within a certain range, and more particularly not to exceed a threshold temperature, to avoid the risk of the Mg ² Si phase nucleating in region A. This is because for a set coating thickness (or a relatively constant diffusion distance between regions A and C), a higher rate of cooling will cause the a-Al phase to grow more rapidly, which will lead to more Mg and Si being rejected in the liquid phase of region A and to a higher enrichment of Mg and Si, or a greater risk of the Mg ² Si phase nucleating, in region A (which is not It is desirable).

On the other hand, for a set cooling rate, a thicker layer (or a thicker local layer region) will increase the diffusion distance between region A and region C, resulting in less of Mg and Si can pass from region A to region C by diffusion within a set time and, in turn, greater enrichment of Mg and Si, or a greater risk of the Mg ² Si phase nucleating , in region A (which is undesirable).

In practice, the Applicant has found that, in order to achieve the distribution of the Mg ² Si particles of the present invention, that is, to avoid the speckle defect on the surface of a coated strip, the cooling rate of the strip Coated coming out of the coating bath has to be in a range of 11-80 ° C / sec. for coating masses up to 75 grams per square meter of strip surface per side and in a range of 11 -50 ° C / sec. for coating masses of 75-100 grams per square meter of strip surface per side. The short range coating thickness variation also has to be controlled to be no more than 40% above the nominal coating thickness within a distance of 5 mm across the strip surface to achieve the distribution of the ^{Mg 2} Si particles of the present invention.

Applicant has also discovered that, when Sr is present in a coating bath, the above-described kinetics of Mg ² Si nucleation can be significantly influenced. At certain concentration levels of Sr, it is strongly segregated into the quaternary alloy layer (ie, the chemistry of the quaternary alloy phase changes). Sr also changes the surface oxidation characteristics of the molten layer, resulting in a thinner surface oxide on the surface of the coating. These changes significantly alter the preferential nucleation sites of the Mg ² Si phase and, as a result, the distribution pattern of the Mg ² Si phase in the direction of the thickness of the coating. In particular, the applicant has discovered that, Sr at concentrations of 250-3000ppm in the coating bath makes it virtually impossible for the Mg ² Si phase to nucleate in the quaternary alloy layer or in the surface oxide, presumably due to the very high level of increase in free energy of the system that would otherwise be generated. Instead, the Mg ² Si phase can only nucleate in the central region of the coating in the thickness direction, resulting in a coating structure that is substantially free of Mg ² Si both in the region of the outer surface of the coating as in the region near the steel surface. Therefore, Sr additions in the 250-3000 ppm range are proposed as one of the effective means to achieve a desired distribution of Mg ² Si particles in a coating.

Claims (2)

1. An Al-Zn-Si-Mg alloy coated steel strip comprising an Al-Zn-Si-Mg alloy coating on a steel strip, with a coating thickness greater than 7 microns and less than 30 microns, with the coating having coating thickness variations of no more than 40% in any 5mm diameter section of the coating, with the alloy consisting of 40 to 60% weight percent aluminum, 40 to 60% zinc, 0.3 to 3% silicon and 0.3 to 10% magnesium as main elements, and optionally more than 250 ppm of strontium and less than 3000 ppm of strontium, and optionally iron, vanadium and chromium, and other elements that they are present as unavoidable impurities, the microstructure of the coating comprising Mg ² Si particles, and the distribution of the Mg ² Si particles being such that there is no more than 10% by weight of Mg ² Si particles on the surface of the coating. 2. A hot dip coating process for forming a corrosion resistant Al-Zn-Si-Mg alloy coating on a steel strip characterized by : passing the steel strip through a hot dip coating bath containing aluminum, zinc, silicon, magnesium, and optionally strontium, iron, vanadium, and chromium and forming an alloy coating on the strip with a coating thickness greater than 7 microns and less than 30 microns and with a coating thickness variation of not more than 40% in any 5 mm diameter section of the coating, the alloy consisting of 40 to 60% by weight of aluminum, 40 to 60% zinc, 0.3 to 3% silicon and 0.3 to 10% magnesium as main elements, optionally more than 250 ppm of strontium and less than 3000 ppm of strontium and optionally iron, vanadium and chromium, and other elements that are present as unavoidable impurities, and with cooling of the coated strip exiting the coating bath during solidification of the coating at a rate of less than 80 ° C / sec. for coating masses up to 75 grams per square meter of strip surface per side and less than 50 ° C / sec. for coating masses of 75-100 grams per square meter of strip surface per side, so the variation of the coating thickness and the cooling rate make the distribution of the Mg ² Si particles such that there is no more than 10% by weight of Mg ² Si particles on the surface of the coating.